The global spread of swine-origin influenza A (H1N1) viruses in humans in April 2009 marked the first influenza pandemic in 41 years. Over 35,000 people were infected with this novel H1N1 virus as of Jun. 15, 2009. Last century, an H1N1 influenza virus also caused the devastating 1918-19 pandemic. In addition, an H1N1 virus derived from swine caused an abortive pandemic in 1976. The 1918 influenza virus caused a mild outbreak in the spring of 1918 and a lethal wave globally in the fall of that year, killing as many as 50 million people worldwide. Although the 2009 swine-origin H1N1 influenza virus was viewed as mild in early 2009, the possibility exists that this virus may mutate and become more virulent by the fall of 2009. Therefore there is an urgent need to develop an effective vaccine to prevent a severe pandemic caused by the 2009 H1N1 viruses.
6:2 reassortant influenza viruses (having the HA and NA segments of swine-origin influenza A (H1N1) viruses and the “backbone” segments of attenuated influenza viruses) useful for vaccine development, both live and killed vaccine have been isolated yet most strains isolated thus far display low titers in eggs. The same isolates infect MDCK cells poorly and form tiny plaques with poor CPE. Additionally, a severe loss of virus potency is observed after virus filtration. Accordingly, the initially isolated H1N1 reassortant influenza virus strains are poor candidates for the development of a vaccine strain.
The present invention provides new and/or newly isolated swine influenza H1 hemagglutinin variants that are capable of use for the production of numerous types of vaccines as well as in research, diagnostics, etc. The present invention further provides methods of improving the replication efficiency of H1 influenza viruses. Numerous other benefits will become apparent upon review of the following.